This invention relates to a radiation detector, and more particularly to a radiation detector using a scintillator array such as positron emission tomography for detecting a location of radiation incident to the scintillation array.
There has been developed a new technical field of nuclear medicine for diagnosing and curing a disease of a human body using radioisotope (RI). As one of techniques which belong to this technical field a positron emission tomography (PET) is used for detecting an emitting location of radiation such as gamma-rays with a scintillator.
The positron emission tomography is a type of nuclear imaging apparatus used especially in medical diagnostics and research imaging. In the positron emission tomography, one type of radioactive compound a drug labeled with a nuclide having positron emission capability ) is administered to a patient or other living organism under surveillance. Positrons are positively charged particles, and are emitted from the nuclide of the radioactive compound as isotope decay within the body. Upon emission, the positron encounters an electron, and both are annihilated. As a result of one annihilation, gamma-rays are generated in the form of two photons which travel in approximate opposite directions (about 180 degrees) to one another. Traditionally, the apparatus is disposed so as to surround the body to accumulate information concerning the lines of travel of the emitted photons at different angles around the body under surveillance and process this information through a computer, whereby a tomographic image of the distribution and concentration of the nuclide is obtained and at the same time is two dimensionally displayed together with a sliced image of the body. In this connection, the PET scanner can observe and quantify biochemical and physiological changes that occur naturally and in disorders in the human body or the like.
An amount of the drug to be administered into the patient or the like is preferably smaller to avoid the influence of the nuclide on the body, however, a smaller amount of the drug causes the emitted radiation to be further reduced in intensity. Accordingly, the radiation detector is required to effectively detect the radiation of weak intensity emitted from the body.
In order to satisfy such a requirement in the art, a gamma-rays detector having an inorganic scintillation crystal such as thallium-activated sodium iodide (NaI(T1)), Bi.sub.4 Ge.sub.3 O.sub.12 (BGO), CsF or the like as a scintillator has been utilized as a radiation detector. The gamma-rays detector comprises plural scintillators arranged one or two-dimensionally for converting gamma-rays emitted from the body to a dispersely-emitting scintillation light, and plural photomultiplier tubes (PMT) optically coupled to the scintillators for converting the scintillation light into electrons and multiplying them. In this gamma-rays detector, the scintillation light emitted from one scintillator of the one- or two-dimensionally arranged scintillator array is distributed to the other scintillators in a predetermined distribution ratio and then guided to the photomultiplier tubes corresponding to the respective scintillators, so that amplified electrical signals having statistical information on an incident position of the gamma rays to the scintillator array (that is, a scintillation location) are outputted from the respective photomultiplier tubes. The position of the gamma-rays incident to the scintillator array (the scintillation location) is statistically determined on the basis of the electrical signals. The gamma-rays detector thus constructed enables a detection resolution to be more enhanced, however, simultaneously causes the scintillation light to be attenuated through a distributing process in which the scintillation light generated in one scintillator is distributed to the other scintillators and a guiding process in which the distributed scintillation lights is guided to the photomultiplier tubes. Accordingly, it has been required for this type of gamma-rays detector to prevent the attenuation of the scintillation light particularly through the distributing process and optimumly carry out the distributing process.
Various types of radiation detectors each utilizing an scintillator array and a photomultiplier tube in combination, for example as shown in FIGS. 1 to 3, have been proposed in order to satisfy the above requirement
The radiation detector as shown in FIG. 1 includes a scintillator array 1 comprising one-dimensionally arranged four scintillators 1.sub.1 to 1.sub.4, and two photomultiplier tubes 2.sub.1 and 2.sub.2 one of which is optically coupled to a half (two scintillators) of the scintillator array 1 and the other of which is optically coupled to the other half (the other two scintillators). The scintillator array 1 is provided with a reflection layer at each of the interfaces (coupling surfaces) 3.sub.1 and 3.sub.3 between the neighboring scintillators 1.sub.1 and 1.sub.2 and between the neighboring scintillators 1.sub.3 and 1.sub.4, and further provided with a predetermined area ratio of a reflection layer and a transmission layer at an interface (coupling surface) 3.sub.2 between the neighbor scintillators 1.sub.2 and 1.sub.3. Accordingly, the scintillators 1.sub.1 and 1.sub.4 are optically separated from the scintillators 1.sub.2 and 1.sub.3 through the reflection layers, respectively, but the scintillators 1.sub.2 and 1.sub.3 are optically coupled through the transmission layer to each other. That is, a scintillation light emitted from one of the four scintillators 1.sub.1 to 1.sub.4 is not distributed (transmitted) to the other scintillators through the reflection layers, while a scintillation light emitted from one of the scintillators 1.sub.2 and 1.sub.3 is distributed (transmitted) through the transmission layer to each other. In this case, a distribution ratio of the scintillation light corresponds to the area ratio of the reflection and transmission layers provided at the interface (coupling surface) 3.sub.2. This radiation detector is described in detail in Japanese Unexamined Published Patent Application No. 62-135787.
The radiation detector as shown in FIG. 2 includes a scintillator having plural grooves (slits) 4 obtained by vertically cutting the scintillator in different depths, and two photomultiplier tubes, one of which is optically coupled to a half of the scintillator 1 and the other of which is optically coupled to the other half. The grooves 4 are provided with reflection agent which serves to guide a scintillation light generated within the scintillator toward the photomultiplier tubes while distributing the scintillation light to the other scintillators and the corresponding photomultiplier tubes in a suitable distribution ratio. This type of radiation detector is described in detail in U.S. Pat. No. 4,749,863, and in "IEEE Transactions on Medical Imaging", Vol. 7, No. 4, 1988, pp 264-272.
The radiation detector as shown in FIG. 3 includes a scintillator array comprising plural scintillators 1.sub.11 to 1.sub.44 which are three-dimensionally arranged, plural photomultiplier tubes 2.sub.1 and 2.sub.2 and a light guide 5, sandwiched between the scintillator array and the photomultiplier tubes, for guiding a scintillation light emitted from the scintillator array to the photomultiplier tubes. The light guide may be used one as shown in "IEEE Transactions on Nuclear Science" Vol. 33, No. 1, February 1986, pp 446-451, pp 460-463. In this type of scintillator array, the interfaces (coupling surfaces) between the respective neighboring scintillators are filled (or coated) with the reflection agent. This type of radiation detector is described in detail in Japanese Unexamined Published Patent Application No. 62-129776, in "IEEE Transactions on Nuclear Science", Vol. 33, No. 1, February 1986 and in "IEEE Transactions on Medical Imaging", Vol. 7, No. 4, December 1988. In place of the flat-plate type light guide, a light guide comprising plural segments each having a complicated shape as described in "IEEE Transactions on Nuclear Science", Vol. NS-34, No. 1, February 1987 may be used to distribute a scintillation light emitted from one of the scintillators to the other scintillators in different distribution ratios.
In all of the radiation detectors as described above, the interfaces (coupling surfaces) between the neighboring scintillators are filled (or coated) with the reflection agent to adjust the distribution of scintillation light between the respective neighboring scintillators. The reflection agent has a capability of reflecting incident light therefrom with no leak, but has a little light-absorption property. The light-absorption of the reflection layer is more remarkable as the scintillator or scintillator array is more minutely segmented into plural scintillator units. Further, particularly in a case where a light guide is provided between the scintillator array and the photomultiplier tubes as shown in FIG. 3, light-absorption also occurs in the light guide, so that the scintillation light is more extremely attenuated before transmitted to the photomultiplier tubes. Such attenuation of the scintillation light emitted from the scintillator array (that is, an optical loss) causes energy resolution and timing resolutions to be lowered, and further causes a discriminating characteristic of each scintillator to be degraded.